1. Field of the Invention
The present invention relates to the recycling of substrates printed with radiation curable inks, more particularly with radiation curable inkjet inks.
2. Description of the Related Art
Printing systems, such as offset and flexography, are being increasingly replaced for packaging applications by industrial inkjet printing systems due to their flexibility in use, e.g. variable data printing allowing last minute advertising changes on the packaging, and due to their enhanced reliability, allowing their incorporation into production lines. Radiation curable inkjet inks are particularly preferred because high quality images can be printed on non-absorbing ink-receivers, such as e.g. glass and plastic packaging materials. For example, WO 2008/064248 (PLASTIPAK) discloses digital printing on a plastic container having a non-planar external surface of a digital image of UV curable ink droplets that vary in diameter from 10 to 200 microns and wherein the droplets of ink range from 200 to 1200 drops per inch.
Currently PET bottles have digitally printed paper and polypropylene labels glued to the bottle with hot melt glue as exemplified EP 2349730 A (PLASTIPAK). This results not only in a large inventory of labels, but also in a more complicated manufacturing process.
More and more food and drink products are being packaged in plastic due to its lighter weight and convenience in comparison to glass. Legislation requires increasing amounts of packaging to be recycled and to minimize waste. In this perspective, the recycling of PET by flotation can be seen as one of the success stories.
Flotation is an efficient, density based sorting method used to separate light, floating components (such as caps and labels) from heavy, sunk components (such as grinded PET bottles). Non-PET components, like UV curable inks, that sink together with PET flakes cause serious processing inefficiencies which limit the use of recycled PET in applications such as bottles, film, sheet and fibre. Therefore combinations of PET with other material types that sink in water should be avoided.
WO 2012/003186 (PLASTIPAK) discloses a recyclable plastic article comprising an external surface with an image printed thereon, the image comprising cured droplets of ink, and the droplets of ink comprising a composition including a removal-promoting additive; wherein the removal-promoting additive causes the cured droplets of ink to separate or loosen from the external surface of the article when the image is exposed to a liquid-based solution. As removal-promoting additive, a hydrophilic monomer like methoxy polyethylene glycol monoacrylate is suggested.
However, WO 2012/003186 (PLASTIPAK) is silent on how a sink float separation can be accomplished.
WO 2012/028216 (KHS) discloses a method for digital printing on containers by generating a print layer forming a multi-colour printed image to an intermediate or base layer, which was applied on an outer surface of a container wall substrate wherein the bond strength or adhesion rates between the substrate and the intermediate or base layer are different from the adhesive strength or the adhesion rate between the base layer and the print layer, characterized in that the different adhesive strength or adhesion rates be adjusted by choice of materials and/or the crosslinking of the base layer and the print layer. Although no chemical details are given, it is suggested that to facilitate the separation of various materials during the recycling process, at least the printing inks used should have a density which is such smaller than the density of the material to be recycled.